Magnetic Resonance Imaging (MRI) scanning has grown into a major diagnostic tool, with MRI instrumentation technology becoming a multi-billion dollar global industry. MRI scanners use magnetic fields and radio waves to form images of the body. To perform an MRI scan, a patient or a body part to be scanned is placed inside a radio frequency (RF) volume coil. The MRI scanner forms a strong magnetic field around the area to be imaged by passing an electric current through the wire loops. While this is happening, other coils in the magnet send and receive radio waves. This triggers protons in the body to align themselves. Once aligned, radio waves are absorbed by the protons, which stimulate spinning. Energy is released after “exciting” the molecules, which in turn emits energy signals that are received by the coil. The received signals are sent to a computer which processes the signals and generates an image. The final product is a 3-D image representation of the area being scanned.
Conventional RF coils, such as loops, birdcage coils, and transverse electromagnetic (TEM) coils are resonant structures tuned to specific Larmor frequencies. By using reactive components to tune a coil to the designated mode, the coil current distribution produces a desirable magnetic field for imaging or spectroscopic applications.
Conventional resonant coils have a limited bandwidth, i.e., the desired coil current distribution appears in the close vicinity of the resonant frequency. Outside a certain frequency range, RF power input is reflected back to the generator due to impedance mismatch. This narrowband feature is typically characterized by the quality factor (Q-factor). A high Q-factor is generally appreciated as an indication of low coil/sample loss. Thus, on the one hand, high-Q coils are preferred for their low coil/sample losses. At the same time, the high Q-factor also indicates the sensitivity of the RF coil to its operating conditions. For high-Q coils, the resonant peak may shift with different loadings, e.g., different phantoms and/or settings inside the scanner bore. This problem is especially pronounced in high-field applications, where the design of transmit coils can be an especially challenging task. For multi-nuclear spectroscopic studies, these issues are aggravated because the design of dual-tuned volume coils is more complex than putting two single-tuned volume coils together.
From the electromagnetic perspective, high-Q resonators are sufficient but not necessary for effective, low-loss transmission. Taking resonant loops as an example, they support a standing-wave current distribution which is the superposition of two currents traveling in opposite directions. If the in-phase superposition of the two traveling currents can result in a low loss, so does each of its components.
The use of traveling-wave structures, which are non-resonant, instead of standing-wave structures has been extensively studied. Many broadband antennas are actually transmission lines that can support traveling waves. Due to the lack of sharp resonant peaks, a broadband antenna requires no specific frequency tuning within its operating bandwidth and is more robust and less sensitive to environmental changes and manufacturing defects.
The use of traveling-wave structures in MRI has also been studied by different groups. It has been demonstrated that is feasible to use the RF shield of an MRI scanner as a waveguide to support traveling waves. The main waveguide structure does not require reactive components for tuning, and a circularly polarized patch antenna was applied for RF excitation and signal reception. This concept has been further extended by using dielectric inserts to reduce impedance mismatch between air/tissue boundaries. A thorough comparison of this waveguide transmitter and the TEM coil has been performed using numerical simulations, and the investigation found that wave attenuation and reflection from body boundaries cause by substantial B1+ inhomogeneity, although a high degree of uniformity was achieved inside an empty scanner bore.
One of the main problems of waveguide-based RF transmitters is the existence of a cutoff frequency for the desired waveguide mode. The scanner bore needs to be large enough so that the cutoff frequency of the desired mode is below the Larmor frequency. Otherwise, electromagnetic waves are evanescent, not traveling. Because the bore size or regular human scanners is barely large enough to support ravels waves at 7T, this type of structure is not suitable for clinical MRIs.
In one approach, the transverse electromagnetic (TEM) mode of a coaxial cable was applied to in order to avoid the cutoff frequency issue. However, the requirement of a center conductor makes this approach difficult for human imaging.
In another approach, a surface transmitter was developed that utilizes the TEM mode supported by two closely spaced parallel conductors. A volume transmitter was later developed for parallel transmission by using an array of such surface transmitters or coils.
Current coils are expensive to manufacture, and also require tuning at very specific and discrete frequencies. Such tuning is expensive, time-consuming to implement, and often limits the applicability of a coil to a specific field strength.